In recent years, more than five hundred million pounds of plastics have been recycled per year in the United States. Nevertheless, plastics continue to account for approximately 20% by volume of municipal solid waste. A major source of solid plastic waste is the "curbside mix" generated by residential households. In particular, soft drink bottles composed primarily of polyethylene terephthalate (PET) constitute a significant fraction of the curbside mix and represent an important recycling opportunity which has been the object of much research and development in the past decade.
Typically, plastic household wastes are chopped or ground to facilitate handling or processing. Contaminants to be separated thus exist as flakes within the recoverable bulk plastic. While some contaminants such as paper may be separated by air classification, other plastics, such as those that make up the polyethylene base cup of beverage bottles, can be separated from relatively denser plastic materials, such as polyester, on the basis of density differences in a water bath.
However, unplasticized polyvinyl chloride (PVC) plastic exhibits a specific gravity very similar to that of PET and cannot be separated from PET on the basis of density.
PET and PVC are found together in most residential waste streams, however. PVC is a popular molding resin material for manufacturing containers for edible oils, household cleaning solutions, detergents, and shampoos. Unfortunately, PET and PVC must not be remelted together because the resulting mixed melt produces hydrochloric acid gas which destroys the properties of the plastic material. A single pound of PVC can spoil several thousand pounds of reprocessed PET product for common reuse, e.g., for food grade container uses. For example, if PVC is present when PET is raised to its melting temperature the PVC will burn and char in a manner that produces gas bubbles in the PET and degrades its color. Thus, recycling of the plastic into other end products is precluded as a practical matter, unless PET and PVC are first separated in a recycling process.
Accordingly, attempts have been made to devise practical methods for separating PVC from PET. U.S. Pat. No. 3,925,200 to Izumi et al. and U. K. Patent No. 1,451,471 to Saitoh et al. describe methods for separating polyvinyl chloride from a mixture of plastics by a flotation process similar to processes utilized previously by the mining industry for separating ores.
In particular, plastics of varying hydrophobicity are placed in an aqueous liquid medium which is agitated by mechanical stirring or by the introduction of air bubbles. Polyvinyl chloride particles that separate out are removed as bottoms from the product aqueous liquid medium inasmuch as the polyvinyl chloride has a somewhat lower hydrophobicity and floatability than other plastics, and has a specific gravity relative to water greater than 1. The aforesaid patents teach that the polyvinyl chloride can be made even more wettable and less floatable by contact with a wetting agent such as a lignin sulfonic acid salt. However, because the process is predicated on sinking the polyvinyl chloride and floating the balance of plastics present, it is not well-suited to separating the plastic wastes produced by modern households that are predominantly polyethylene terephthalate. Currently, only a small fraction (about 3% by weight) of the curbside mix is polyvinyl chloride, thus the rest of the household plastic waste (about 97%) would have to be floated.
U.S. Pat. No. 4,617,111 to Grimm et al. describes a method for separating a mixture of polyvinyl chloride and polyethylene terephthalate particles. A low-density solvent is applied to all of the particles but is preferentially absorbed by the polyvinyl chloride particles in an amount sufficient to substantially lessen their density. The less dense particles are alleged to be separable from the polyethylene terephthalate particles by flotation or centrifugation. The low-density solvent, which is applied as part of an aqueous sodium hydroxide solution, has a density of less than 0.95 grams per centimeter and may be an alcohol, ketone, ester, or ether. The polyvinyl chloride separated by the method necessarily contains a significant amount of the solvent. As these solvents are explosive, extreme danger exists during processing. Accordingly, the method is unattractive from a practical standpoint. The additional prospect of subsequently having to separate the solvent for reuse makes the method even less attractive to many recycling processors.
U.S. Pat. No. 4,031,039 to Mizumoto et al. proposes a method for treating a mixture of polymers including polyvinyl chloride by sequentially contacting the mixture with various solvents that are capable of completely dissolving one or more of the polymers. The individual polymers are later recovered from the solvent by fractionation. The method is relatively complicated and requires energy.
In conjunction with the separation methods described above, many recycling processors visually inspect the solid waste before it is chopped or ground and remove as many whole PVC containers as they can find. More recently, some robotic devices equipped with sensors for detecting polyvinyl chloride have been employed to help humans with this laborious task. Inevitably, polyvinyl chloride containers slip through undetected and contaminate the recovered polyethylene terephthalate. The current invention provides a method to recover plastic thus contaminated as well as a reusable product.